1. Field of the Invention
The invention is directed to a vacuum insulated panel comprised of a filler and a vacuum-tight high-barrier film envelope with an air-permeable sheet element that serves as a filter material for dust, particularly for the dust-tight sealing of an evacuation opening, and to a method for producing such a vacuum insulated panel.
2. Description of the Prior Art
Evacuated insulation panels consist of a high-thermal-conductivity, open-pored core that is able to withstand compressive loads, and a completely closed, plastic film envelope having a high barrier effect against gases and water vapor. The core is evacuated at a gas pressure of between 0.01 mbar and 5 mbar. This reduces the thermal conductivity of the residual gases in the porous core to a minimum. Depending on the core material, the thermal conductivity in the evacuated state can be between 0.0015 and 0.010 W/mK. Commonly used core materials are powder, open-pore foam, glass fibers or aerogels. Foams, glass fibers or powder usually come in board form and are cut to the desired size, wrapped in high-barrier film and pumped free of air in a vacuum chamber.
In evacuating powdered core materials, care must be taken that no powder dust finds its way out of the pouch to be evacuated and into the vacuum chamber. In German Patent Specification DE 100 58 566, for example, it is recommended to use non-woven polyester to wrap a core panel made of pyrogenic silicic acid pressed into board form. This prevents powder dust from the inside from contaminating the sealing seam of the film pouch at the outlet opening during evacuation. Contamination with minute particles of powder can substantially degrade the barrier effect of the sealing seam placed at the opening of the film pouch after the completion of the evacuation process, and can completely destroy the leaktightness of the vacuum insulated panel and let air in. A disadvantage of this method is that the non-woven polyester has to be folded elaborately so that it fits the core snugly even at the edges of the panel. The sealing seam of the high-barrier film is preferably sealed near the edges of the core panel to avoid producing any unnecessarily long, protruding flaps on the sides. If, on the other hand, portions of the non-woven envelope fabric deviate from the edge, there is a risk that this fabric will be co-sealed into the seam, thus, under normal circumstances, causing the seam to leak at the points concerned.
An air-permeable woven or non-woven fabric can also be used in another production variant. Here, a pouch is first made from the air-permeable sheet element, powder is fed in, the pouch is sealed, and the structure is pressed into board form. This board is then wrapped in barrier film or slid into a pouch made of high-barrier film. Here again, there is a risk that, in the presence of close-fitting sealing seams, some non-woven or woven material will get into the seam and cause a leak.
In a further production variant, which is known from DE 10 2005 045 726, a pouch is first formed from high barrier film by suitable cutting and welding, and a powder from which the core will subsequently be molded is fed directly into this pre-formed pouch made of high-barrier film. A trough-shaped filter felt is glued into the opening of this pre-formed pouch. This keeps the powder in the pouch during evacuation in the vacuum chamber, and prevents the film seams and the chamber from being contaminated. Another option is to take a pouch that is closed on two opposite sides and seal the bottom, third side with filter felt before filling the pouch. Filling then takes place through the remaining opening on the fourth side, which is sealed with another sealing seam after filling is completed. Evacuation is then performed through the side sealed with the filter felt. However, applying the polyester felt to the inside of the pouch in a manner that ensures powder-tightness at all points is a relatively elaborate operation in both versions. Since the shape of the pouch is already fixed before the filter felt is inserted, the sealing seams of the high-barrier film that are adjacent the remaining opening in the pouch have already been made, and the filter felt cannot inadvertently get into these seams, which would render the system non-leaktight, since any sealed-in filter felt will seriously disrupt the tightness of a given sealing seam. Due to the existing shape of the pouch, a high-edged, trough-shaped structure must be used as the filter felt, which, first of all, is elaborate to make, and must be glued to the flat and sides of the high-barrier pouch by the sealing layer provided inside the latter. The gluing of the polyester felt to the inner sealing layer of the pouch is usually done by thermal fusion. However, this does not produce any sealing seams that can be accessed from the outside, as when the pouch is sealed, but only inner welds that cannot be designated as sealing seams in the sense of the invention because they do not join the two inwardly disposed sealing surfaces of two high-barrier film regions directly to each other, but instead each join only a single region of the high-barrier film to a trough-shaped, pre-formed filter felt. Even during the final sealing of the still air-permeable region of the pouch opening by a last sealing seam, no portion of the filter felt must get into this sealing seam of the barrier pouch, since the pouch would then immediately lose its leaktightness. Production of this kind would thus require high expenditure and extreme care in order to avoid an increased number of rejects.
From the disadvantages of the described prior art comes the problem initiating the invention, that of improving a vacuum insulated panel of the aforesaid species so that contamination of the sealing surface(s) with inadvertently entrained powder or dust particles during evacuation can be reliably prevented with minimal expenditure.